1. Field of the Invention
This invention relates to a data transmission system for an automotive vehicle, which comprises a plurality of electronic control units installed on the vehicle and a contmon communication line connecting the electronic control units with each other.
2. Prior Art
Recently, automotive vehicles are electronically controlled in respect of most part of the engine control, the suspension control, and so on. Accordingly, they are each equipped with a data transmission system for connecting a plurality of electronic control units (hereinafter referred to as "the ECU's") for the respective types of control by a common communication line (hereinafter referred to as "the network bus") to transmit data necessary for the above types of control to each other by the use of asynchronous serial data signals.
High reliability is demanded of this type of data transmission system for automotive vehicles, and therefore, the system employs a twisted-pair transmission method in which the network bus is implemented by a composite signal line formed by a twisted-pair transmission line having a pair of signal lines to which a sending end (ECU) delivers digital data signals having opposite polarities (i.e. being opposite in phase), respectively, so that a receiving end (ECU) can receive a message from the sending end based on changes in the polarities of the signals indicative of the logical states of the signals. The twisted-pair transmission method makes it possible to transmit data even if one of the pair of signal lines is disconnected, thereby enhancing reliability of the system.
However, the twisted-pair transmission method is capable of transmitting data even if one of the twisted pair is disconnected, for example, as described above. Therefore, it is neither possible to recognize occurrence of the fault nor to determine which of them is faulty due to disconnection, ground fault, or the like, which prevents the system from recovering from the fault. Therefore, when the other of the twisted-pair, which has been safe, becomes faulty, it is entirely impossible for the system to perform data transmission.
Further, within such a conventional data transmission system, a receiving end (ECU) is required to perform synchronization of bit times by making the phase of a bit time of a serial data signal received from the network bus and the phase of a bit time within the receiving end synchronous with each other, as well as to perform resynchronization by temporarily lengthen or shorten the bit time within the receiving end to correct a difference in phase of the bit time within the receiving end from the bit time of the received serial data signal with which the bit time within the receiving end was already made synchronous, so as to always ensure correct demodulation of the serial data signal.
Conventionally, as shown in FIG. 8a and FIG. 8b, the resynchronization is permitted only when the logical state of the serial data signal has changed within each synchronizing time period (see FIG. 8b at a time point t1), to prevent the system from being susceptible to noises.
However, there is usually a slight difference between the reference clock frequency of a sending end (ECU) and that of a receiving end (ECU), so that when the serial data signal continues to be in the same logical state, the difference can be accumulated to such an extent as will cause a point of change in the logical state of the serial data signal to fall outside a corresponding resynchronizing time period (see FIG. 8b at a time point t3), causing the bit time of the receiving end to be out of phase.
If each synchronizing time period is set to a longer duration for prevention of such a problem, there now arises an inconvenience of a higher possibility of erroneous resynchronization caused by noises or the like.